1. Field of the Invention
The present invention relates to an electrode material for a lithium secondary battery that comprises a powder of particles comprising silicon as a major component, an electrode structure comprising the electrode material and a secondary battery comprising the electrode structure.
2. Related Background Art
Recently, it has been said that because the amount of CO2 gas contained in the air is increasing, global warming may be occurring due to the greenhouse effect. Thermal power plants use fossil fuels to convert a thermal energy into an electric energy, however they exhaust a large amount of CO2 gas, thereby making it difficult to newly construct thermal power plants. Accordingly, for effective use of an electric power generated in thermal power plants, the so-called load leveling approach has been proposed wherein an electric power generated at night, which is an excess power, may be stored in a household secondary battery or the like, whereby the stored electric power can be used during the daytime when electric power consumption increases.
In addition, the development of a high energy-density secondary battery has been demanded for electric vehicles that do not exhaust air pollutants such as COx, NOx, and hydrocarbons. Further, the development of compact, lightweight, high performance secondary batteries is urgently demanded for applications in portable electrical equipment such as notebook personal computers, video cameras, digital cameras, mobile phones, PDAs (Personal Digital Assistant) or the like.
As such a lightweight, compact secondary battery, a rocking chair type battery referred to as “lithium ion battery” which, during a charging reaction, uses a lithium intercalation compound as a positive electrode substance for allowing lithium ions to be deintercalated from between layers thereof and uses a carbonaceous material represented by graphite as a negative electrode substance for allowing lithium ions to be intercalated between planar layers of a 6-membered network-structure formed of carbon atoms have been developed and partly put into practical use.
However, with this “lithium ion battery”, because the negative electrode formed of a carbonaceous material can theoretically intercalate only a maximum of ⅙ of a lithium atom per one carbon atom, a high energy-density secondary battery comparable with a lithium primary battery when using metallic lithium as a negative electrode material has not been realized.
If an amount of lithium more than the theoretical amount is tried to be intercalated in a negative electrode comprising carbon of a “lithium ion battery” during charging or charging is performed under a high current density condition, there is a possibility that lithium metal may grow in a dendrite shape on the carbon negative electrode surface, resulting in an internal short-circuit between the negative and the positive electrodes due to repeated charge/discharge cycles, so that any “lithium ion battery” which has a capacity more than the theoretical capacity of a graphite negative electrode has not provided a sufficient cycle life.
On the other hand, a high-capacity lithium secondary battery that uses metal lithium for a negative electrode has been drawing attention but not put in practical use yet.
This is because the charge/discharge cycle life is very short. This short charge/discharge cycle life is considered to be ascribed to the fact that metal lithium reacts with impurities such as water or organic solvents contained in the electrolyte to form an insulating film or that the surface of a metallic lithium foil is not flat and has a portion at which an electric field is concentrated, whereby repeated charging/discharging causes lithium to grow in a dendrite shape, resulting in an internal short-circuit between the negative and positive electrodes, thereby leading to the end of the battery life.
In order to suppress the progress of the reaction in which metal lithium reacts with water or organic solvents contained in the electrolyte, which is a problem peculiar to the secondary battery using a metal lithium negative electrode, a method which uses a lithium alloy containing lithium, aluminum and the like as a negative electrode has been proposed.
However, this method is not currently in wide practical use because the lithium alloy is too hard to wind in a spiral form, and therefore a spiral-wound type cylindrical battery cannot be made, because the cycle life is not sufficiently long, and because an energy density comparable to that of a battery using metal lithium for a negative electrode cannot sufficiently be obtained.
In order to resolve the above-mentioned problems, heretofor, U.S. Pat. Nos. 6,051,340, 5,795,679, and 6,432,585, Japanese Patent Application Laid-Open Nos. 11-283627 and 2000-311681 and International Publication WO 00/17949 have proposed a secondary battery that uses a negative electrode for a lithium secondary battery comprised of elemental tin or silicon.
U.S. Pat. No. 6,051,340 has proposed a lithium secondary battery that uses a negative electrode comprising an electrode layer formed of a metal that is alloyable with lithium such as silicon or tin and a metal that is not alloyable with lithium on a current collector of a metal material that is not alloyable with lithium.
U.S. Pat. No. 5,795,679 proposes a lithium secondary battery using a negative electrode formed of a powder of an alloy of an element such as nickel or copper with an element such as tin. U.S. Pat. No. 6,432,585 proposes a lithium secondary battery that uses a negative electrode with an electrode material layer containing 35% or more by weight of particles comprised of silicon or tin with a average particle diameter of 0.5 to 60 μm and having a void ratio of 0.10 to 0.86 and a density of 1.00 to 6.56 g/cm3.
Japanese Patent Application Laid-Open No. 11-283627 proposes a lithium secondary battery that uses a negative electrode comprising silicon or tin having an amorphous phase; Japanese Patent Application Laid-Open No. 2000-311681 proposes a lithium secondary battery that uses a negative electrode comprising amorphous tin-transition metal alloy particles with a non-stoichiometric composition; and International Publication WO 00/17949 proposes a lithium secondary battery using a negative electrode comprising amorphous silicon-transition metal alloy particles with a non-stoichiometric composition.
However, in the lithium secondary batteries according to the above-mentioned proposals, the efficiency of the electricity amount involved in lithium release relative to the electricity amount involved in a first lithium insertion does not reach the same level of performance as a graphite negative electrode, so that further improvement in the efficiency have been expected. In addition, since the resistances of the electrodes of the lithium secondary batteries of the above proposals are higher than that of a graphite electrode, lowering in resistance has been desired.
Japanese Patent Application Laid-Open No. 2000-215887 proposes a high-capacity, high charging/discharging efficiency lithium secondary battery in which a carbon layer is formed on the surface of particles of a metal or semi-metal which is alloyable with lithium, in particular silicon particles, through chemical vapor disposition using thermal decomposition of benzene or the like to improve electrical conductivity, thereby suppressing volume expansion when alloying with lithium to prevent breakage of an electrode.
However, with this lithium secondary battery, while the theoretical charge capacity calculated for Li4.4Si as a silicon/lithium compound is 4200 mAh/g, an electrode performance allowing lithium insertion/release of an electricity amount exceeding 1000 mAh/g has not been attained, so that development of a high-capacity, long life negative electrode has been desired.